The present invention relates generally to an image reading method and apparatus, and more particularly to such an image reading method and apparatus for setting the read signal value, to be outputted when reading a highlight point and shadow point of an original, with the consideration of the density value (or concentration value) of a desirable color chip or the density value of a desirable point on an orignal picture.
Recently, in the electronic plate-making fields to principally contemplate to produce a printed matter, a high-quality image can be obtained in accordance with the computer image processing technique. On the other hand, a reading apparatus for reading a high-quality image can be required for producing the high-quality image. A conventional reading method will be described hereinbelow. In a conventional image reading apparatus, the read image signal and the original density value are coordinated or corresponded with each other by using a plurality of points on a color chip or an original, before reading the original. As the coordinating system, there is a system where the output signal values obtained when reading a color chip having a known density are set to be coordinated with the respective density values of the color chip, thereby heightening the reliability of the read image signal. Further, in order for performing the coordination between the original density value and the read image signal by using a color on the original, there is provided a system where as disclosed in the Japanese Patent Provisional Publication No. 61-274231 the operator designates a plurality of points on an original so that the output signal values obtained when reading these points are set to become operator-desired values by using a color chip.
Generally, color image reading apparatus use signals obtained by separating an original picture into a plurality of colors (for example, R, G and B). Further, in monitors or printers for displaying the image signals, color values of respective signals that have been color separated and are coincident with each other are treated as gray. Accordingly, in order for faithfully reproducing the original picture, the balance of the plural colors color-separated is required to be determined so that the values of the color signals obtained by separating the gray are coincident with each other. There is a problem which arises with the conventional image reading apparatus, however, in that difficulty is encountered to meet the requirement that the operator freely and easily sets the coordination between the read image and the original density value with the plural colors color-separated being balanced with each other. For instance, if using the system where the read signal value of a color chip having a predetermined density value is set so as to coordinated with the density value of the color chip, the coordination between the read image signal and the original density value can be determined with the balance of the plurality of colors color-separated being kept, while the coordination cannot be made freely by the operator.
On the other hand, according a method such as disclosed in the Japanese Patent Provisional Publication No. 61-274231, after the density values of the color components of a plurality of desirable points on an original picture are measured using an ordinal densitometer, the read signal values of the plurality of points are determined with reference to the measured density values so that the coordination between the reading image and the original density value is effected by using a color chip having density values corresponding to the color components representative of the plurality of points. In this case, although the balance of the plurality of colors color-separated can be kept, at least one of the density values corresponding to the color components at the plurality of points is required to be coincident with the density value of the color chip, and hence limitation is imposed upon the selection from the plural points and further the selection is difficult. In addition, since a color chip having a number of density is required to be prepared, searching the target density value becomes troublesome. Moreover, the ordinal is required to be prepared in addition to the image reading apparatus.